1. Technical Field
The present invention relates to a steering apparatus for a vehicle provided with an angular ratio varying device which can vary a ratio of a steering angle of a steerable vehicle wheel with respect to an operational angle of a steering member.
2. Description of Related Art
Steering of a vehicle is carried out by operating a steering mechanism in accordance with the operation of a steering member such as a steering wheel and varying a steering angle of a steerable vehicle wheel by the operation of the steering mechanism. In recent years, as a steering apparatus for a vehicle to carry out such a steering, one that is configured so as to be able to vary an angular ratio (=a steering angle/an operational angle) of the steering angle of the steerable vehicle wheel with respect to the operational angle of the steering member has been known (for example, refer to Japanese Patent Application Laid-Open No. 2003-252227).
The steering apparatus for a vehicle described in Japanese Patent Application Laid-Open No. 2003-252227 is a separation-type steering apparatus for a vehicle, namely, a so-called steer-by-wire system steering apparatus for a vehicle, which is configured in such a manner that a steering member to be operated by an operator and a steering mechanism operating so as to steer a steerable vehicle wheel are mechanically separated; attaching a steering motor to the steering mechanism, the steering motor is operated based on the operational direction and the operational angle of the steering member; and the steering force is added to the steering mechanism to carry out steering, namely, a so-called Steer-by-Wire System of steering apparatus for a vehicle. This steering apparatus for a vehicle can freely vary the angular ratio of the steering angle with respect to the operational angle without being mechanically restricted by operating the steering mechanism driving the steering motor.
According to the steering apparatus for a vehicle of the above-described Japanese Patent Application Laid-Open No. 2003-252227, a steering control unit imports an operational angle of the steering wheel that is detected by an operational angle sensor (Paragraph [0023]) and imports a real steering angle (a steering angle) of a wheel for steering to be steered that is detected by a real steering angle sensor (Paragraph [0020]). A steering angle control unit obtains a target steering angle by multiplying the imported operational angle by a predetermined control gain. The steering angle control unit carries out steering control operation by a feedback control based on a deviation between the target steering angle and the real steering angle. The control gain is determined as a value that is decreased depending on increase of a speed of a car or a value that is decreased depending on increase of a swing degree of a vehicle to be decided by a yaw rate or a lateral acceleration (Paragraphs [0026] and [0027]). As described above, in the steering apparatus for a vehicle of the above-described Japanese Patent Application Laid-Open No. 2003-252227, the angle ratio of the real steering angle (the steering angle) with respect to the operational angle can be varied by control of the steering angle control unit.
Varying a steering property in accordance with a running condition such as a difference in height of a speed of a car and a measure of the operational angle by varying the angular ratio of the steering angle with respect to the operational angle, the steering apparatus for a vehicle of the above-described Japanese Patent Application Laid-Open No. 2003-252227 can carry out steering suitable for the running condition. In addition, by varying only the steering angle of the steerable vehicle wheel, it is also possible to carry out active steering such as a motion control on a slip and avoidance of a danger of an obstacle.
However, in the steering apparatus for a vehicle as in the above-described Japanese Patent Application Laid-Open No. 2003-252227, in a case where an abnormality is caused in any of a control device for variably controlling an angular ratio and various sensors to be used for this control to be made into a fail condition, it is feared that steering is interfered by an inappropriate control to be carried out under this condition. Therefore, the steering apparatus for a vehicle in the above-described Japanese Patent Application Laid-Open No. 2003-252227 is configured so as to be capable of carrying out manual steering by arranging an electromagnetic clutch between the steering member and the steering mechanism and mechanically connecting the steering member to the steering mechanism due to the engagement operation of the electromagnetic clutch in the fail condition.